What is needed, therefore, is a system and method for consolidating powders which will avoid the problems encountered by the techniques used in the past.
It is, therefore, a primary object to provide a system and method for using relatively long duration, relatively low current density, proximately constant voltage electrical current flow through the particulate material during the consolidation process.
Another object of the invention is to provide a system and method for consolidating particulate material using relatively long duration, relatively low current density in a manner that will permit achievement of ninety-eight percent (98%) or greater of the material's theoretical density, even when used with materials which traditionally have been very difficult to consolidate, such as stainless steel, Sendust, 4405 and the like.
Another object of the invention is to provide a system and method for avoiding undesired arcing at the interface between the punch and particulate material, thereby improving the useful life of the punches.
A particulate materials consolidation system comprising: a particulate material die for receiving a particulate material to be consolidated; a first punch and a second punch which cooperate with said particulate material die to compress the particulate material; a power source coupled to said first and second punches to energize said particulate material to a predetermined energy level for a duration of at least 0.1 second at a current of less than about 10 KA/cmwhen said particulate material is being consolidated; and a feedback control coupled to said punches and said power source for monitoring a characteristic of said particulate material when it is being consolidated and generating a feedback signal in response thereto; said power source adjusting said predetermined energy level in response to said feedback signal while said particulate material is being consolidated such that said particulate material achieves at least 95 percent of its maximum theoretical density. The particulate material consolidation system as recited in claim 1 wherein said power source comprises a power supply which energizes said particulate material for a duration of less than 1 second. The particulate material consolidation system as recited in claim 2 wherein said first and second punches comprise a punch resistivity of less than about 25×10ohm-meter. The particulate material consolidation system as recited in claim 1 wherein said particulate material die comprises a die surface having an insulator thereon. The particulate material consolidation system as recited in claim 5 wherein said insulator is ceramic. The particulate material consolidation system as recited in claim 5 wherein said insulator is a coating integral with said die surface. The particulate material consolidation system as recited in claim 5 wherein said insulator is a coating comprises a thickness of less than about 6×10meter. The particulate material consolidation system as recited in claim 7 wherein said coating comprises an oxide or a diamond film. The particulate material consolidation system as recited in claim 1 wherein said power source comprises a DC power source. The particulate material consolidation system as recited in claim 1 wherein said power source comprises an AC power supply. The particulate material consolidation system as recited in claim 1 wherein said feedback control comprises a voltage sensor coupled to said first and second punches for measuring a voltage across said particulate material and for generating a voltage signal which defines said feedback signal. Field of the Invention This invention relates to a method and apparatus for consolidating particulate material, such as powders, and more particularly, to a system and method for consolidating particulate material by applying relatively long duration current flow at relatively low current densities to the particulate material in order to achieve densities in excess of ninety percent (90%) of the theoretical maximum density for the particulate material.2. A problem with this approach has been, that under these conditions electrical arcing may occur at the interface between the powder and the current-conducting punches.
Still another object of the invention is to provide a consolidation system and method which realizes only modest temperature rises in the powder during the consolidation process.Yet another object of the invention is to provide a consolidation system and method which utilizes active feedback control of the power input during the consolidation process, thereby permitting tailoring of the power input to the particulate material being consolidated.This invention relates to a system and method for consolidating particulate material, such as particulate material, in order to achieve at least ninety-five percent (95%) or even ninety-eight percent (98%) of its maximum theoretical density using a relatively long duration, relatively low current density current flow through the material. One of the major limitations of the powder material compaction process is that, with most materials, less than full densification is achieved during the compaction process. 4,929,415; 4,975,412; 5,084,088; 5,529,746; 5,380,473 are examples of consolidation techniques of the type used in the past. Still another problem of the prior art is that the walls of the molds or dies used during the consolidation process required an insulator, such as ceramic.In one embodiment, the consolidation system includes a feedback control for monitoring various characteristics associated with the particulate material being consolidated and providing feedback information to a power supply which controls the amount of current supplied to the particulate material in order to achieve the desired density. Typically, powder material consolidation results in less than ninety-three percent (93%) of its full theoretical density for many powders and for difficult to compact materials (such as stainless steel) less than eighty-five percent (85%) of theoretical density is achieved. For example, Okazaki discloses a method for sintering and forming powder. One significant problem with this approach is that the ceramic used for insulating the walls were not suitable for generating parts having shapes which require intricate details because when the intricate details are machined into the ceramic insulators and the insulators in the die, the ceramic would sometimes crack or chip upon use during the consolidation process.The consolidation system and method is characterized in that the duration of the current is greater than 0.1 second, but typically less than about 1 second, while the current is less than about 10KA/cm International Search Report completed Feb. Less than full density, results in degraded material properties, such as strength, stiffness, magnetisity and the like. This method uses a high voltage of 3 KV or more which is applied to a mold filled with the powder using an electrode which maintains a high current of 50 KAcmor greater for a period of time from 10 to 500 microseconds. Another problem with prior art techniques is that they did not permit tailoring of the power input to the powder mass to allow controlled power input.
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Marcus et al., University of Texas at Austin, Journal of Metals, Dec. Description of Related Art The consolidation of particulate material under relatively high compaction pressure using molds and dies to manufacture parts has become a frequently used industrial process. 4,975,412 also discloses a method of processing superconducting materials which utilizes, again, a high voltage and current density to provide sharp bonding between or among the particulate material. This arcing will severely limit the useful life of the punches and, therefore, must be overcome in order to make this technique commercially viable.